Additive fabrication processes for producing three dimensional objects are well known. Additive fabrication processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts. These three-dimensional parts may be formed from liquid resins, powders, or other materials.
A well-known, non-limiting example of an additive fabrication process is stereolithography (SL). Stereolithography is a process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser that traces a pattern of a cross section through a liquid radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser traces another cross section to harden another layer of resin on top of the previous layer. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured, and is called a “green model.” Although not required, the green model may be subjected to post-curing to enhance the mechanical properties of the finished part. An example of an SL process is described in, for example, U.S. Pat. No. 4,575,330.
Lasers have traditionally served as the radiation source of choice in additive fabrication processes such as stereolithography. The use of gas lasers to cure liquid radiation curable resin compositions is well known. The delivery of laser energy in a stereolithography system can be Continuous Wave (CW) or Q-switched pulses. CW lasers provide continuous laser energy and can be used in a high speed scanning process. Historically, several types of lasers have been used in stereolithography, with peak spectral outputs ranging traditionally from 193 nm to 355 nm in wavelength, although other wavelength variants exist. The light emitted from lasers is monochromatic, that is, a high percentage of the total spectral output occurs within a very narrow wavelength range. Amongst laser-based additive manufacturing systems in the industry, those operating at a peak spectral output of 355 nm have become most prevalent.
Laser-based systems, particularly those operating at a peak spectral output of at or near 355 nm, are not without their drawbacks, however. The significant power output of such laser-based systems sometimes generates excess heat at the point of irradiation which may be detrimental to the resin. Further, the use of a laser at any wavelength requires scanning point-by-point on the resin surface, a process which can be particularly time-consuming when the cross-sectional pattern to be cured is complex. Also, 355 nm laser-based systems are expensive and are associated with high maintenance costs and energy consumption.
To combat some of the drawbacks associated with laser-based systems, other additive fabrication systems have begun to utilize image projection technology as the source of actinic radiation. One example of this is Liquid Crystal Display (LCD), a technology that is well-known in other industries such as the manufacture of television sets and computer monitors. Another non-limiting example was developed by Texas Instruments called Digital Light Processing (DLP®). DLP systems selectively transfer light from an input source and project that light in a desired output pattern or mask using pixel-representative microscopic mirrors that are controlled by and affixed to a microchip known as a Digital Micromirror Device (DMD). DLP technology was developed to be used in image projection systems as an alternative display system to LCD-based technology. The exceptional image sharpness, brightness, and uniformity associated with DLP systems lends it well to additive fabrication wherein image resolution and precision is critical, as the boundaries of the light projected ultimately define that of the three-dimensional object to be cured and created. Furthermore, image projection systems such as LCD and DLP provide a theoretical speed advantage in that they enable an entire cross-sectional layer to be exposed and cured simultaneously. Furthermore, wherein the required cure time in laser-based systems is proportional to the complexity of the cross-section to be scanned, image projection systems are said to be cross-section independent, meaning the exposure time of a given layer does not change with increasing shape complexity of any given layer. This makes them particularly well-suited for parts created via additive fabrication with complex and detailed geometries.
DLP and LCD are not alternative methods of producing light itself; rather they provide a way of processing the light emanating from existing light sources into a more desirable pattern. Thus, coupled input light sources are also still needed. Although the light input to an image projection system may be from any source, including traditional lamps or even lasers, more commonly the input light is collimated from one or more light emitting diodes (LEDs).
LEDs are semiconductor devices which utilize the phenomenon of electroluminescence to generate light. At present, LED light sources for additive fabrication systems emit light at wavelengths between 300 and 475 nm, with 365 nm, 375 nm, 395 nm, 401 nm, 405 nm, and 420 nm being common peak spectral outputs. See textbook, “Light-Emitting Diodes” by E. Fred Schubert, 2nd Edition, © E. Fred Schubert 2006, published by Cambridge University Press, for a more in-depth discussion of LED light sources. LEDs provide the advantage that they theoretically operate at close to peak efficiency for a longer duration than other light sources. Furthermore, they are typically more energy efficient and inexpensive to maintain, resulting in a lower initial and ongoing cost of ownership than laser-based optics systems.
Therefore, various additive fabrication systems have employed one of the following non-limiting examples of optics configurations: (1) laser only, (2) laser/DLP, (3) LED only, (4) LED/DLP, or (5) LED/LCD. Systems not utilizing DLP technology may also incorporate other collimating or focusing lenses/mirrors to selectively direct the light onto the liquid resin.
Recently, newer additive fabrication systems—regardless of the optics configuration—have begun to more frequently employ light sources that emit radiation at wavelengths greater than the traditional output at 355 nm. Others have shifted away from monochromatic light sources, opting instead for those which emit light possessing a wider spectral output distribution. Thus, such newer systems incorporating laser/DLP-, LED-, LED/DLP-, or LED/LCD-based optics configurations have begun operate at peak spectral outputs of a longer wavelength and with wider spectral distributions than was previously common. The wavelengths employed therein have shifted away from 355 nm towards the direction of the visible spectrum, with some even possessing a peak spectral output into the visible range. Such longer wavelengths (i.e. from about 375 nm to about 500 nm) are heretofore referred to as “UV/vis”.
Some non-limiting reasons commonly cited for the current trend towards the increasing use of optics in the UV/vis region are: (1) the reduced cost (both initial and maintenance costs) of light sources that operate in the UV/vis range, as well as (2) the fact that UV/vis light sources emit radiation at lower energies than light sources emitting deeper into the UV region, and, all else being equal, are less damaging to human tissue. This makes them less harmful upon accidental exposure than those operating deeper into the UV region. As the popularity of additive manufacturing continues to grow amongst the consumer, “prosumer”, and industrial market segments, the need for additive fabrication systems employing lower-cost, less dangerous sources of actinic radiation with which to cure liquid photopolymers will become increasingly important.
The benefits of the utilization of a UV/vis light source/optics system are not without notable tradeoffs, however. To date, the largest drawback is the relatively increased difficulty with which to develop suitable photopolymers for systems utilizing UV/vis optics. One of the primary reasons for this is that in addition to the natural phenomenon of reduced energy of light at longer wavelengths, the intensity of commercial light sources also typically decreases as the wavelength of the peak spectral output increases. Thus, whereas the traditional 355 nm laser-based lights systems might impart an irradiance of 1500 W/cm2 at the surface of a resin, known commercial systems operating at around 400 nm impart an irradiance of roughly only around 1/1000th of that value at the surface of a resin. In fact, the irradiance at the resin surface imparted by the UV/vis optics on existing 365 nm or 405 nm DLP-based commercial additive fabrication systems can be as low as 0.1 W/cm2 or even 0.0002 W/cm2 for some more economical desktop units. These relatively reduced radiation energies/intensities make it more difficult for photopolymerization reactions to occur in the radiation-curable resins via such UV/vis optics unless exposure times become prohibitively long. This in turn increases part build times significantly, such that the theoretical speed advantage of photomasking display systems is negated. Furthermore, fewer photoinitiating systems—in particular cationic photoinitiating systems—exist on the market for promoting photopolymerization at such longer UV/vis wavelengths.
The aforementioned challenges have resulted in a limited number of photopolymers being made available for additive fabrication systems employing the modern optics systems operating in the UV/vis region, relative to the variety of options available for systems operating deeper into the UV region, such as 355 nm laser-based systems.
Radically-polymerizable resins are known to exist for systems employing UV/vis optics. Such resins generally consist of one or more (meth)acrylate compounds (or other free-radical polymerizable organic compounds) along with a free-radical photoinitiator for radical generation. U.S. Pat. No. 5,418,112 describes one such radical-curable system. Although radically-polymerizable resins will readily cure under even the relatively lower energy and lower intensity afforded by UV/vis optics, they are not suitable for all additive fabrication applications. First, (meth)acrylate based resins considered suitable for additive fabrication processes have traditionally produced cured parts with insufficient mechanical properties to be incorporated into many end-use applications. Therefore, they produce parts which are typically not robust enough for non-prototyping applications. Also, such resins typically exhibit problems of deformation, such as production of warped or malformed parts, because of residual strain due to the differential shrinkage during curing. Such problems are exacerbated on larger-platform additive fabrication machines, wherein the cumulative differential shrinkage effect amplifies part warping or malformation as cured objects become larger. These problems of deformation can be partially rectified through software which accounts for known shrinkage rates by modifying the CAD file from which a solid three-dimensional part is generated. However, software corrections are insufficient to completely compensate for deformation in parts which have intricate and complicated shapes, or require a strict dimensional tolerance across long distances.
Another well-known type of resin suitable for use in additive fabrication systems is a “hybrid” curable resin, or one that comprises: (1) epoxies, oxetanes, or other types of cationically polymerizable compounds; (2) one or more cationic photoinitiators; (3) acrylate resins or other types of free radical polymerizable compounds; and (4) one or more free radical photoinitiators. Examples of such hybrid curable systems are described in, for example, U.S. Pat. No. 5,434,196. Such resins have long-been known to result in cured parts produced via additive fabrication processes with superior mechanical properties relative to all-acrylate based resins. Furthermore, hybrid curable systems are superior to all-acrylate systems in that they suffer less from the differential shrinkage problems which have long-plagued all-acrylate systems.
However, because the ring-opening process of cationic polymerization generally occurs more slowly and requires more activation energy than free radical polymerization, it is inherently more difficult to ensure that such formulations for additive fabrication applications adequately cure, or successfully “build” a three dimensional object. And even if curing does at least partially occur after the hybrid-curable resin is subjected to actinic radiation, the green model produced therefrom possesses insufficient mechanical strength (or “green strength”) as measured by, for example, modulus of elasticity or fracture strength, to be used in many additive manufacturing applications. Such problems are significantly exacerbated by UV/vis optics that emit radiation at lower energies and intensities than conventional systems.
Because of limitations with regards to cationic polymerization, heretofore, no known hybrid liquid radiation curable resin for additive fabrication exists that is suitable for more modern additive fabrication systems employing UV/vis optics. Furthermore, no liquid radiation curable resin for additive fabrication exists—hybrid curable or otherwise—that is suitable for additive fabrication systems employing UV/vis optics and also is simultaneously (1) sufficiently fast-curing, (2) able to impart sufficient mechanical strength, and (3) resistant to shrinkage or warping, thereby preventing deformation into the three dimensional parts from which it is cured.
From the foregoing, it is evident that a heretofore unmet need exists to provide hybrid curable, liquid radiation resin compositions that are suitable for use in additive fabrication systems employing UV/vis optics that can produce three-dimensional parts with mechanical properties at least equal to existing hybrid-curable materials designed for traditional, laser-based 355 nm systems.